1. Field of the Invention
This invention relates to a color interface circuit for a video monitor which produces color images in response to signals supplied by a digital processor, and particularly to such an interface circuit which enables a monitor operating in a standard format of primary color analog input signals, such as Red, Green and Blue, to display colors signified by digital signals which are in various other color formats.
2. Description of the Related Art
Information produced by a digital processor is commonly displayed on the screen of a color CRT to permit visual monitoring of such information. The input terminals of the CRT monitor require signals which control the relative intensities of each of a set of three primary colors on the monitor screen, the standard set or format of primary colors being Red, Green and Blue. An intensity ("I") signal is also frequently added to the color signals to control their relatively purity with respect to a reference gray level. Since the processor produces digital output signals, digital-to-analog conversion is required to obtain corresponding analog signals for application to the monitor input terminals. This is one of the functions of a display interface circuit between the processor and the monitor. Certain kinds of processors, for example the type 5150 (RGBI) of IBM Corporation, produce digital signals respective bits of which, with few exceptions, respectively correspond to the individual R, G, B primary colors of the standard video monitor color format. Other processors, however, such as the type IIe (XRGB) of Apple Computer Corporation, produce digital signals which completely depart from the standard primary color format and are intended for a monitor specifically adapted to operate in the format of that particular processor. With such processors, respective bits of the digital signal identify colors which are actually combinations of different proportions of the standard R, G and B primary colors. Consequently, it has not been possible to employ the same monitor for such processors as well as for processors operating in the standard B, G and R color format. Suppliers of different kinds of digital processors have therefore had to maintain in inventory almost an equal number of matching display monitors, and purchasers of particular processors have been limited in the choice of monitors available to them.
Although it is technically feasible to devise interface circuits which translate the digital output signals of a particular processor from one color format to another, circuits capable of translating from any of a variety of different color formats have been found to be inordinately complicated and more expensive than simply providing alternative monitors having color formats matching the processors with which they are to be used.